Pneumatic tire having tread with hydroxy-terminated polybutadiene

ABSTRACT

The present invention is directed to a pneumatic tire comprising a tread, the tread comprising a rubber composition comprising a diene elastomer, a pre-hydrophobated precipitated silica, a blocked mercaptosilane, a terpene-phenol resin, and a low molecular weight polybutadiene functionalized with a hydroxyl functional group, wherein the pre-hydrophobated precipitated silica is hydrophobated prior to its addition the rubber composition by treatment with at least one silane selected from the group consisting of alkylsilanes, alkoxysilanes, organoalkoxysilyl polysulfides and organomercaptoalkoxysilanes and optionally at least one dispersing aid selected from the group consisting of fatty acids, diethylene glycols, polyethylene glycols, fatty acid esters of hydrogenated or non-hydrogenated C 5  or C 6  sugars, polyoxyethylene derivatives of fatty acid esters of hydrogenated or non-hydrogenated C 5  or C 6  sugars.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to have good wet skid resistance, lowrolling resistance, and good wear characteristics. It has traditionallybeen very difficult to improve a tire's wear characteristics withoutsacrificing its wet skid resistance and traction characteristics. Theseproperties depend, to a great extent, on the dynamic viscoelasticproperties of the rubbers utilized in making the tire.

In order to reduce the rolling resistance and to improve the treadwearcharacteristics of tires, rubbers having a low hysteresis havetraditionally been utilized in making tire tread rubber compounds. Onthe other hand, in order to increase the wet skid resistance of a tire,rubbers which undergo a large energy loss have generally been utilizedin the tire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads. For instance,various mixtures of styrene-butadiene rubber and polybutadiene rubberare commonly used as a rubbery material for automobile tire treads.However, improvements in rolling resistance often occur in tandem with areduction in wet traction, and vice versa. There is a continuing need,therefore, to develop tread having both good rolling resistance and wettraction.

SUMMARY OF THE INVENTION

The present invention is directed to a pneumatic tire comprising atread, the tread comprising a rubber composition comprising a dieneelastomer, a pre-hydrophobated precipitated silica, a blockedmercaptosilane, a terpene-phenol resin, and a low molecular weightpolybutadiene functionalized with a hydroxyl functional group, whereinthe pre-hydrophobated precipitated silica is hydrophobated prior to itsaddition the rubber composition by treatment with at least one silaneselected from the group consisting of alkylsilanes, alkoxysilanes,organoalkoxysilyl polysulfides and organomercaptoalkoxysilanes andoptionally at least one dispersing aid selected from the groupconsisting of fatty acids, diethylene glycols, polyethylene glycols,fatty acid esters of hydrogenated or non-hydrogenated C₅ or C₆ sugars,polyoxyethylene derivatives of fatty acid esters of hydrogenated ornon-hydrogenated C₅ or C₆ sugars.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed a pneumatic tire comprising a tread, the treadcomprising a rubber composition comprising a diene elastomer, apre-hydrophobated precipitated silica, a blocked mercaptosilane, aterpene-phenol resin, and a low molecular weight polybutadienefunctionalized with a hydroxyl functional group, wherein thepre-hydrophobated precipitated silica is hydrophobated prior to itsaddition the rubber composition by treatment with at least one silaneselected from the group consisting of alkylsilanes, alkoxysilanes,organoalkoxysilyl polysulfides and organomercaptoalkoxysilanes andoptionally at least one dispersing aid selected from the groupconsisting of fatty acids, diethylene glycols, polyethylene glycols,fatty acid esters of hydrogenated or non-hydrogenated C₅ or C₆ sugars,polyoxyethylene derivatives of fatty acid esters of hydrogenated ornon-hydrogenated C₅ or C₆ sugars.

In one embodiment, the rubber composition comprises as the dieneelastomer, from 70 to 90 phr of at least one styrene-butadiene rubber,and from 10 to 30 phr of a natural rubber or synthetic polyisoprene.

In one embodiment, the styrene-butadiene rubber comprises a firststyrene-butadiene rubber and a second styrene-butadiene rubber.

In one embodiment, at least one of the first and secondstyrene-butadiene rubber is functionalized with a alkoxysilane group andat least one group selected from sulfur containing functional group andprimary amino functional groups.

In one embodiment as diene elastomer for the rubber composition, thediene elastomer includes A) from 40 to 60 phr of a firststyrene-butadiene rubber having a Tg ranging from −70° C. to −5° C. andfunctionalized with a alkoxysilane group and sulfur containingfunctional group, B) from 20 to 30 phr of a second styrene-butadienerubber containing from 25 to 45 percent by weight of styrene, a vinyl1,2 content of 20 to 60 percent by weight based on the rubber weight, aTg of from −30° C. to −5° C., and C) from 10 to 30 phr of a naturalrubber or synthetic polyisoprene. Alternatively, the firststyrene-butadiene rubber has a Tg ranging from −40 to −10° C.

In one embodiment, the rubber composition includes from 40 to 60 phr ofa first styrene-butadiene rubber functionalized with an alkoxysilanegroup and a functional group selected from sulfur containing functionalgroups and amino functional groups. Suitable sulfur containing groupsinclude thiol, thioether, thioester, sulfide, or sulfanyl group.Suitable amino functional groups include primary, secondary, andtertiary amino groups. Additional examples of rubbers which may be usedinclude solution polymerized styrene-butadiene functionalized withgroups such as alkoxy including monoalkoxy, dialkoxy, and trialkoxy,silyl, thiols, thioester, thioether, sulfanyl, mercapto, sulfide, andcombinations thereof. Such functionalized solution polymerized polymersmay be functionalized at the polymer chain ends for example viafunctional initiators or terminators, or within the polymer chains forexample via functional monomers, or a combination of in-chain andend-of-chain functionalization. Specific examples of suitable functionalsolution polymerized polymers include those described in U.S. Pat. Nos.8,217,103 and 8,569,409 having alkoxysilyl and sulfide (i.e. thioether)functionality. Such thiol functionality includes thiol or sulfanylfunctionality arising from cleavage of sulfur containing groups duringcompound processing, such as for example from thioesters and thioethers.

In one embodiment, the styrene-butadiene rubber is obtained bycopolymerizing styrene and butadiene, and characterized in that thestyrene-butadiene rubber has a thiol group and an alkoxysilyl groupwhich are bonded to the polymer chain. In one embodiment, thealkoxysilyl group is an ethoxysilyl group.

The thiol group may be bonded to any of a polymerization initiatingterminal, a polymerization terminating terminal, a main chain of thestyrene-butadiene rubber and a side chain, as long as it is bonded tothe styrene-butadiene rubber chain. However, the thiol group ispreferably introduced to the polymerization initiating terminal or thepolymerization terminating terminal, in that the disappearance of energyat a polymer terminal is inhibited to improve hysteresis losscharacteristics. The thiol group may further exist as a blocked thiol(also known as blocked mercapto group) having a protective functionalgroup attached to the sulfur atom such as in a thioester or thioether,which is then cleaved to expose the thiol sulfur during rubber mixing.

Further, the content of the alkoxysilyl group bonded to the polymerchain of the (co)polymer rubber is preferably from 0.5 to 200 mmol/kg of(styrene-butadiene rubber. The content is more preferably from 1 to 100mmol/kg of styrene-butadiene rubber, and particularly preferably from 2to 50 mmol/kg of styrene-butadiene rubber.

The alkoxysilyl group may be bonded to any of the polymerizationinitiating terminal, the polymerization terminating terminal, the mainchain of the (co)polymer and the side chain, as long as it is bonded tothe (co)polymer chain. However, the alkoxysilyl group is preferablyintroduced to the polymerization initiating terminal or thepolymerization terminating terminal, in that the disappearance of energyis inhibited from the (co)polymer terminal to be able to improvehysteresis loss characteristics.

The styrene-butadiene rubber can be produced by polymerizing styrene andbutadiene in a hydrocarbon solvent by anionic polymerization using anorganic alkali metal and/or an organic alkali earth metal as aninitiator, adding a terminating agent compound having a primary aminogroup protected with a protective group and/or a thiol group protectedwith a protecting group and an alkoxysilyl group to react it with aliving polymer chain terminal at the time when the polymerization hassubstantially completed, and then conducting deblocking, for example, byhydrolysis or other appropriate procedure. In one embodiment, thestyrene-butadiene rubber can be produced as disclosed in U.S. Pat. No.7,342,070. In another embodiment, the styrene-butadiene rubber can beproduced as disclosed in WO 2007/047943.

In one embodiment, the solution polymerized styrene-butadiene rubber isas disclosed in WO 2007/047943 and is functionalized with analkoxysilane group and a blocked thiol, and comprises the reactionproduct of a living anionic polymer and a silane-sulfide modifierrepresented by the formula (R⁴O)_(x)R^(4y)Si—R⁵—S—SiR⁴ ₃ wherein Si issilicon; S is sulfur; O is oxygen; x is an integer selected from 1, 2and 3; y is an integer selected from 0, 1, and 2; x+y=3; R⁴ is the sameor different and is (C₁-C₁₆) alkyl; and R⁵ is aryl, and alkyl aryl, or(C₁-C₁₆) alkyl. In one embodiment, R⁵ is a (C₁-C₁₆) alkyl. In oneembodiment, each R⁴ group is the same or different, and each isindependently a C₁-C₅ alkyl, and R⁵ is C₁-C₅ alkyl.

The solution polymerized styrene-butadiene rubber has a glass transitiontemperature in a range from −70° C. to −5° C., alternatively from −40 to−10° C. A reference to glass transition temperature, or Tg, of anelastomer or elastomer composition, where referred to herein, representsthe glass transition temperature(s) of the respective elastomer orelastomer composition in its uncured state or possibly a cured state ina case of an elastomer composition. A Tg can be suitably determined as apeak midpoint by a differential scanning calorimeter (DSC) at atemperature rate of increase of 10° C. per minute, for example accordingto ASTM D7426 or equivalent.

Suitable styrene-butadiene rubbers functionalized with an alkoxysilanegroup and a thiol group are available commercially, such as Sprintan SLR4602 from Trinseo.

In one embodiment, the rubber composition contains from 20 to 30 phr ofa second styrene-butadiene rubber, wherein the second styrene-butadienerubber is solution-polymerized styrene-butadiene rubber (SSBR) with abound styrene content of from 25 to 45 percent by weight, a vinyl 1,2content of from 20 to 60 percent by weight based on the rubber weight,and a Tg of from about −30° C. to about −5° C. As the secondstyrene-butadiene rubber, suitable solution polymerizedstyrene-butadiene rubbers may be made, for example, by organo lithiumcatalyzation in the presence of an organic hydrocarbon solvent. Thepolymerizations employed in making the rubbery polymers are typicallyinitiated by adding an organolithium initiator to an organicpolymerization medium that contains the monomers. Such polymerizationsare typically carried out utilizing continuous polymerizationtechniques. In such continuous polymerizations, monomers and initiatorare continuously added to the organic polymerization medium with therubbery polymer synthesized being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem. Suitable polymerization methods are known in the art, forexample as disclosed in U.S. Pat. Nos. 4,843,120; 5,137,998; 5,047,483;5,272,220; 5,239,009; 5,061,765; 5,405,927; 5,654,384; 5,620,939;5,627,237; 5,677,402; 6,103,842; and 6,559,240.

As the second styrene-butadiene rubber, suitable solution polymerizedstyrene-butadiene rubbers are available commercially, such as TufdeneE680 SSBR from Asahi Chemical, F3438 from LG Chem, and the like. Suchsolution polymerized styrene-butadiene rubber may be tin- orsilicon-coupled, as is known in the art. In one embodiment, suitableSSBR may be at least partially silicon-coupled.

In one embodiment, the rubber composition contains from 10 to 30 phr ofa natural rubber or synthetic polyisoprene. Such synthetic cis1,4-polyisoprene and cis 1,4-polyisoprene natural rubber are well knownto those having skill in the rubber art.

The term “phr” as used herein, and according to conventional practice,refers to “parts by weight of a respective material per 100 parts byweight of rubber, or elastomer.”

The rubber composition also contains from 3 to 20 phr of a blockedmercaptosilane include blocked forms of mercapto alkylalkoxysilanes,such as mercaptopropyl triethoxysilane, mercaptopropyl trimethoxysilane,mercaptopropyl methyldimethoxysilane, mercaptopropyl methyldiethoxysilane, mercaptopropyl dimethymethoxysilane, mercaptoethyltriethoxysilane, and mercaptopropyl tripropoxysilane. In each case ablocking group may be bonded to the mercapto sulfur, such blocking groupform thioesters —C(═O)—C_(n)H_(2n+1), where n is from 1 to 10,thioethers, or silylsulfide groups. In one embodiment, the blockinggroup is a octanoyl group forming a thioester, and the blockedmercaptosilane is S-octanoylmercaptopropyltriethoxysilane (otherwiseknown as 3-octanoylthio-1-propyltriethoxysilane) available at NXT fromMomentive.

The rubber composition includes a functionalized liquid polymer.

Suitable liquid polymer should have double bonds that can react withsulfur and the polymer matrix to form cross-links. Suitable liquidpolymers are derived from conjugated diolefin (or diene) monomers. Suchliquid polymers can also contain repeat units which are derived fromother monomers which are copolymerizable with conjugated diolefinmonomers. For instance, the liquid polymer can also contain repeat unitswhich are derived from vinyl aromatic monomers, such as styrene.Polybutadiene rubber, polyisoprene rubber, styrene-butadiene rubber,isoprene-butadiene rubber, styrene-isoprene rubber andstyrene-isoprene-butadiene rubber are some representative examples ofpolymers which can be used as the liquid polymer.

The liquid polymers are functionalized with at least one functionalgroup including alkoxysilyl, hydroxyl, epoxy groups, amino, carboxyl,maleic groups, and maleimide groups. The liquid polymers may befunctionalized at the polymer chain ends for example via functionalinitiators or terminators, or within the polymer chains for example viafunctional monomers, or a combination of in-chain and end-of-chainfunctionalization.

The liquid polymers are low molecular weight rubbery polymers ofconjugated diolefin monomers. These low molecular weight rubberypolymers will also typically be comprised of repeat units which arederived from one or more conjugated diolefin monomers. Such lowmolecular weight rubbers can also, of course, contain repeat units whichare derived from other monomers which are copolymerizable withconjugated diolefin monomers. For instance, the low molecular weightrubbery polymer can contain repeat units which are derived from vinylaromatic monomers, such as styrene. Low molecular weight polybutadienerubber, low molecular weight polyisoprene rubber, low molecular weightstyrene-butadiene rubber, low molecular weight isoprene-butadienerubber, low molecular weight styrene-isoprene rubber and low molecularweight styrene-isoprene-butadiene rubber are some representativeexamples of low molecular weight rubbery polymers which can be modifiedto make the wetting agents of this invention. The low molecular weightrubbery polymer will typically have a weight average molecular weightwhich is within the range of about 1000 to about 25,000 g/gmol. The lowmolecular weight rubbery polymer will more typically have a weightaverage molecular weight which is within the range of about 2000 toabout 15,000 g/gmol.

The weight average molecular weight Mw may be measured with gelpermeation chromatography (GPC) using polystyrene calibration standards,such as is done according to ASTM 3536. GPC is a well-known methodwherein polymers are separated according to molecular size, the largestmolecule eluting first. The chromatograph is calibrated usingcommercially available polystyrene molecular weight standards. Thedetector used is preferably an ultraviolet detector. The fraction ofchains existing as mono chains is determined as the ratio of the areasunder the GPC curve, i.e., (mono chain peak area)/(total area).

In one embodiment, the rubber compositions include from 3 to 30 phr,alternatively, 3 to 10 phr of functionalized liquid polymer.

In one embodiment, the rubber composition includes from 3 to 30 phr of apolybutadiene functionalized with primary hydroxyl groups at eachterminus and having a molecular weight Mw ranging from 1000 to 25000g/gmol, alternatively 2000 to 4000 g/gmol, and a Tg ranging from −50° C.to −20° C. In one embodiment, the hydroxyl functionalized polybutadieneis Krasol LBH-P 2000 from Cray Valley.

The rubber composition includes from 3 to 25 phr of a hydrocarbon resinor a terpene phenol resin having a Tg ranging from 50 to 120 C. Asuitable measurement of Tg for resins is DSC according to ASTM D6604 orequivalent.

The terpene phenol is generally described as the reaction product of aphenol and a terpene. The terpene monomer as the raw material of theterpene phenol resin is not particularly limited. It is preferable thatthe terpene monomer is a monoterpene hydrocarbon such as α-pinene andlimonene. From the standpoint of the excellent balance between the lossproperty and the rigidity, raw monomers comprising α-pinene are morepreferable, and α-pinene is most preferable.

As the terpene phenol resin described above, resins of various gradesare available as commercial products such as “YS POLYSTER” and“MIGHTYACE G” manufactured by YASUHARA CHEMICAL Co., Ltd.

Representative hydrocarbon resins include coumarone-indene-resins,petroleum resins, C₅/C₉ resins, terpene polymers, alphamethyl styreneresins and mixtures thereof.

Coumarone-indene resins are well known. Various analysis indicate thatsuch resins are largely polyindene; however, typically contain randompolymeric units derived from methyl indene, coumarone, methyl coumarone,styrene and methyl styrene.

Suitable petroleum resins include both aromatic and nonaromatic types.Several types of petroleum resins are available. Some resins have a lowdegree of unsaturation and high aromatic content, whereas some arehighly unsaturated and yet some contain no aromatic structure at all.Differences in the resins are largely due to the olefins in thefeedstock from which the resins are derived. Conventional derivatives insuch resins include dicyclopentadiene, cyclopentadiene, their dimers anddiolefins such as isoprene and piperylene. Copolymers of these monomerwith one another or with aromatic such as styrene and alphamethylstyrene are also included.

In one embodiment the resin is an aromatic modifiedpolydicyclopentadiene.

Terpene polymers are commercially produced from polymerizing a mixtureof alpha or beta pinene in mineral spirits. The resin is usuallysupplied in a variety of melting points ranging from 10° C. to 135° C.

In one embodiment, the resin is derived from styrene andalphamethylstyrene. It is considered that, in one aspect, its glasstransition temperature (Tg) characteristic combined with its molecularweight (Mn) and molecular weight distribution (Mw/Mn) provides asuitable compatibility of the resin in the rubber composition, thedegree of compatibility being directly related to the nature of therubber composition.

The presence of the styrene/alphamethylstyrene resin with a rubber blendwhich contains the presence of the styrene-butadiene elastomer isconsidered herein to be beneficial because of observed viscoelasticproperties of the tread rubber composition such as complex and storagemodulus, loss modulus tan·delta and loss compliance at differenttemperature/frequency/strain as hereinafter generally described.

The properties of complex and storage modulus, loss modulus, tan·deltaand loss compliance are understood to be generally well known to thosehaving skill in such art. They are hereinafter generally described.

The molecular weight distribution of the resin is visualized as a ratioof the resin's molecular weight average (Mw) to molecular weight numberaverage (Mn) values and is considered herein to be in a range of about1.5/1 to about 2.5/1 which is considered to be a relatively narrowrange. This believed to be advantageous because of the selectivecompatibility with the polymer matrix and because of a contemplated useof the tire in wet and dry conditions over a wide temperature range.

The glass transition temperature Tg of the copolymer resin is consideredherein to be in a range of about 20° C. to about 100° C., alternativelyabout 30° C. to about 80° C., depending somewhat upon an intended use ofthe prepared tire and the nature of the polymer blend for the tiretread. A suitable measurement of TG for resins is DSC according to ASTMD6604 or equivalent.

The styrene/alphamethylstyrene resin is considered herein to be arelatively short chain copolymer of styrene and alphamethylstyrene witha styrene/alphamethylstyrene molar ratio in a range of about 0.40 toabout 1.50. In one aspect, such a resin can be suitably prepared, forexample, by cationic copolymerization of styrene and alphamethylstyrenein a hydrocarbon solvent.

Thus, the contemplated styrene/alphamethylstyrene resin can becharacterized, for example, by its chemical structure, namely, itsstyrene and alphamethylstyrene contents and softening point and also, ifdesired, by its glass transition temperature, molecular weight andmolecular weight distribution.

In one embodiment, the styrene/alphamethylstyrene resin is composed ofabout 40 to about 70 percent units derived from styrene and,correspondingly, about 60 to about 30 percent units derived fromalphamethylstyrene. In one embodiment, the styrene/alphamethylstyreneresin has a softening point according to ASTM No. E-28 in a range ofabout 80° C. to about 145° C.

Suitable styrene/alphamethylstyrene resin is available commercially asResin 2336 from Eastman or Sylvares SA85 from Arizona Chemical.

The rubber composition may also include from 10 to 25 phr of processingoil. Processing oil may be included in the rubber composition asextending oil typically used to extend elastomers. Processing oil mayalso be included in the rubber composition by addition of the oildirectly during rubber compounding. The processing oil used may includeboth extending oil present in the elastomers, and process oil addedduring compounding. Suitable process oils include various oils as areknown in the art, including aromatic, paraffinic, naphthenic, vegetableoils, and low PCA oils, such as MES, TDAE, SRAE and heavy naphthenicoils. Suitable low PCA oils include those having a polycyclic aromaticcontent of less than 3 percent by weight as determined by the IP346method. Procedures for the IP346 method may be found in Standard Methodsfor Analysis & Testing of Petroleum and Related Products and BritishStandard 2000 Parts, 2003, 62nd edition, published by the Institute ofPetroleum, United Kingdom.

In the rubber composition, the sum of the amounts of the resin, the lowmolecular weight polybutadiene, and the processing oil ranges from 10 to45 phr.

Also included in the rubber composition is from 50 to 150 phr ofpre-hydrophobated precipitated silica. By pre-hydrophobated, it is meantthat the silica is pretreated, i.e., the pre-hydrophobated precipitatedsilica is hydrophobated prior to its addition to the rubber compositionby treatment with at least one silane. Suitable silanes include but arenot limited to alkylsilanes, alkoxysilanes, organoalkoxysilylpolysulfides and organomercaptoalkoxysilanes.

In an alternative embodiment, the pre-hydrophobated precipitated silicamay be pre-treated with a silica coupling agent comprised of, forexample, an alkoxyorganomercaptoalkoxysilane or combination ofalkoxysilane and organomercaptoalkoxysilane prior to blending thepre-treated silica with the rubber instead of reacting the precipitatedsilica with the silica coupling agent in situ within the rubber. Forexample, see U.S. Pat. No. 7,214,731.

The prehydrophobated precipitated silica may optionally be treated witha silica dispersing aid. Such silica dispersing aids may include glycolssuch as fatty acids, diethylene glycols, polyethylene glycols, fattyacid esters of hydrogenated or non-hydrogenated C₅ or C₆ sugars, andpolyoxyethylene derivatives of fatty acid esters of hydrogenated ornon-hydrogenated C₅ or C₆ sugars.

Exemplary fatty acids include stearic acid, palmitic acid and oleicacid.

Exemplary fatty acid esters of hydrogenated and non-hydrogenated C₅ andC₆ sugars (e.g., sorbose, mannose, and arabinose) include, but are notlimited to, the sorbitan oleates, such as sorbitan monooleate, dioleate,trioleate and sesquioleate, as well as sorbitan esters of laurate,palmitate and stearate fatty acids. Exemplary polyoxyethylenederivatives of fatty acid esters of hydrogenated and non-hydrogenated C₅and C₆ sugars include, but are not limited to, polysorbates andpolyoxyethylene sorbitan esters, which are analogous to the fatty acidesters of hydrogenated and non-hydrogenated sugars noted above exceptthat ethylene oxide groups are placed on each of the hydroxyl groups.

The optional silica dispersing aids if used are present in an amountranging from about 0.1% to about 25% by weight based on the weight ofthe silica, with about 0.5% to about 20% by weight being suitable, andabout 1% to about 15% by weight based on the weight of the silica alsobeing suitable.

For various pre-treated precipitated silicas see, for example, U.S. Pat.Nos. 4,704,414, 6,123,762 and 6,573,324.

Suitable pre-hydrophobated silica is available commercially for examplefrom PPG as the Agilon series.

In addition to the pre-hydrophobated silica, the rubber composition mayinclude an untreated, or non-prehydrophated precipitated silica.

The commonly employed siliceous pigments which may be used in the rubbercompound include conventional pyrogenic and precipitated siliceouspigments (silica). In one embodiment, precipitated silica is used. Theconventional siliceous pigments employed in this invention areprecipitated silicas such as, for example, those obtained by theacidification of a soluble silicate, e.g., sodium silicate.

Such conventional silicas might be characterized, for example, by havinga BET surface area, as measured using nitrogen gas. In one embodiment,the BET surface area may be in the range of about 40 to about 600 squaremeters per gram. In another embodiment, the BET surface area may be in arange of about 80 to about 300 square meters per gram. The BET method ofmeasuring surface area is described in the Journal of the AmericanChemical Society, Volume 60, Page 304 (1930).

The conventional silica may also be characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, alternatively about 150 to about 300.

The conventional silica might be expected to have an average ultimateparticle size, for example, in the range of 0.01 to 0.05 micron asdetermined by the electron microscope, although the silica particles maybe even smaller, or possibly larger, in size.

Various commercially available silicas may be used, such as, only forexample herein, and without limitation, silicas commercially availablefrom PPG Industries under the Hi-Sil trademark with designations 210,243, etc; silicas available from Solvay, with, for example, designationsof Z1165MP, Z165GR and Zeosil Premium 200MP and silicas available fromDegussa AG with, for example, designations VN2 and VN3, etc.

Commonly employed carbon blacks can be used as a conventional filler inan amount ranging from 1 to 10 phr. Representative examples of suchcarbon blacks include N110, N121, N134, N220, N231, N234, N242, N293,N299, N315, N326, N330, N332, N339, N343, N347, N351, N358, N375, N539,N550, N582, N630, N642, N650, N683, N754, N762, N765, N774, N787, N907,N908, N990 and N991. These carbon blacks have iodine absorptions rangingfrom 9 to 145 g/kg and DBP number ranging from 34 to 150 cm³/100 g.

In one embodiment, the rubber composition may optionally contain aconventional sulfur containing organosilicon compound. In oneembodiment, the sulfur containing organosilicon compounds are the3,3′-bis(trimethoxy or triethoxy silylpropyl) polysulfides. In oneembodiment, the sulfur containing organosilicon compounds are3,3′-bis(triethoxysilylpropyl) disulfide and/or3,3′-bis(triethoxysilylpropyl) tetrasulfide.

The amount of the optional sulfur containing organosilicon compound in arubber composition will vary depending on the level of other additivesthat are used. Generally speaking, the amount of the compound will rangefrom 0.5 to 20 phr. In one embodiment, the amount will range from 1 to10 phr.

It is readily understood by those having skill in the art that therubber composition would be compounded by methods generally known in therubber compounding art, such as mixing the various sulfur-vulcanizableconstituent rubbers with various commonly used additive materials suchas, for example, sulfur donors, curing aids, such as activators andretarder, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidantsand antiozonants and peptizing agents. As known to those skilled in theart, depending on the intended use of the sulfur vulcanizable andsulfur-vulcanized material (rubbers), the additives mentioned above areselected and commonly used in conventional amounts. Representativeexamples of sulfur donors include elemental sulfur (free sulfur), anamine disulfide, polymeric polysulfide and sulfur olefin adducts. In oneembodiment, the sulfur-vulcanizing agent is elemental sulfur. Thesulfur-vulcanizing agent may be used in an amount ranging from 0.5 to 8phr, alternatively with a range of from 1.5 to 6 phr. Typical amounts ofprocessing aids comprise about 1 to about 50 phr. Typical amounts ofantioxidants comprise about 1 to about 5 phr. Representativeantioxidants may be, for example, diphenyl-p-phenylenediamine andothers, such as, for example, those disclosed in The Vanderbilt RubberHandbook (1978), Pages 344 through 346. Typical amounts of antiozonantscomprise about 1 to 5 phr. Typical amounts of fatty acids, if used,which can include stearic acid comprise about 0.5 to about 3 phr.Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typicalamounts of waxes comprise about 1 to about 5 phr. Often microcrystallinewaxes are used. Typical amounts of peptizers comprise about 0.1 to about1 phr. Typical peptizers may be, for example, pentachlorothiophenol anddibenzamidodiphenyl disulfide.

Accelerators are used to control the time and/or temperature requiredfor vulcanization and to improve the properties of the vulcanizate. Inone embodiment, a single accelerator system may be used, i.e., primaryaccelerator. The primary accelerator(s) may be used in total amountsranging from about 0.5 to about 4, alternatively about 0.8 to about 1.5,phr. In another embodiment, combinations of a primary and a secondaryaccelerator might be used with the secondary accelerator being used insmaller amounts, such as from about 0.05 to about 3 phr, in order toactivate and to improve the properties of the vulcanizate. Combinationsof these accelerators might be expected to produce a synergistic effecton the final properties and are somewhat better than those produced byuse of either accelerator alone. In addition, delayed actionaccelerators may be used which are not affected by normal processingtemperatures but produce a satisfactory cure at ordinary vulcanizationtemperatures. Vulcanization retarders might also be used. Suitable typesof accelerators that may be used in the present invention are amines,disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides,dithiocarbamates and xanthates. In one embodiment, the primaryaccelerator is a sulfenamide. If a second accelerator is used, thesecondary accelerator may be a guanidine, dithiocarbamate or thiuramcompound.

In one embodiment, the rubber compositions may include from 1 to 10 phras a vulcanization modifier an α,ω-bis(N,N′-dihydrocarbylthiocarbamamoyldithio)alkane. Suitable α,ω-bis(N,N′-dihydrocarbylthiocarbamamoyldithio)alkanes include1,2-bis(N,N′-dibenzylthiocarbamoyl-dithio)ethane;1,3-bis(N,N′-dibenzylthiocarbamoyldithio)propane;1,4-bis(N,N′-dibenzylth-iocarbamoyldithio)butane;1,5-bis(N,N′-dibenzylthiocarbamoyldithio)pentane;1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane;1,7-bis(N,N′-dibenzylthiocarbamoyldithio)heptane;1,8-bis(N,N′-dibenzylthiocarbamoyldithio)octane;1,9-bis(N,N′-dibenzylthiocarbamoyldithio)nonane; and1,10-bis(N,N′-dibenzylthiocarbamoyldithio)decane. In one embodiment, thevulcanization modifier is1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane available as Vulcurenfrom Bayer.

The mixing of the rubber composition can be accomplished by methodsknown to those having skill in the rubber mixing art. For example, theingredients are typically mixed in at least two stages, namely, at leastone non-productive stage followed by a productive mix stage. The finalcuratives including sulfur-vulcanizing agents are typically mixed in thefinal stage which is conventionally called the “productive” mix stage inwhich the mixing typically occurs at a temperature, or ultimatetemperature, lower than the mix temperature(s) than the precedingnon-productive mix stage(s). The terms “non-productive” and “productive”mix stages are well known to those having skill in the rubber mixingart. The rubber composition may be subjected to a thermomechanicalmixing step. The thermomechanical mixing step generally comprises amechanical working in a mixer or extruder for a period of time suitablein order to produce a rubber temperature between 120° C. and 190° C. Theappropriate duration of the thermomechanical working varies as afunction of the operating conditions, and the volume and nature of thecomponents. For example, the thermomechanical working may be from 1 to20 minutes.

The rubber composition may be incorporated in a variety of rubbercomponents of the tire. For example, the rubber component may be a tread(including tread cap and tread base), sidewall, apex, chafer, sidewallinsert, wirecoat or innerliner. In one embodiment, the component is atread.

The pneumatic tire of the present invention may be a race tire,passenger tire, aircraft tire, agricultural, earthmover, off-the-road,truck tire, and the like. In one embodiment, the tire is a passenger ortruck tire. The tire may also be a radial or bias.

Vulcanization of the pneumatic tire of the present invention isgenerally carried out at conventional temperatures ranging from about80° C. to 200° C. In one embodiment, the vulcanization is conducted attemperatures ranging from about 110° C. to 180° C. Any of the usualvulcanization processes may be used such as heating in a press or mold,heating with superheated steam or hot air. Such tires can be built,shaped, molded and cured by various methods which are known and will bereadily apparent to those having skill in such art.

This invention is illustrated by the following examples that are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

Example 1

In this example, the effect of a hydroxy-terminated polybutadiene on theperformance of a tread compound is illustrated. Rubber compositions weremixed in a multi-step mixing procedure following the recipes in Table 1,with all amounts given in phr. Standard amounts of curatives were alsoincluded. Rubber compounds were then cured and tested for rollingresistance (RR) and wet braking performance, with results given in Table2.

TABLE 1 Sample No. Reference Example 1 Example 2 BR¹ 10 0 0 SBR² 40 0 0SBR³ 50 27 27 SBR⁴ 0 52 53 Natural Rubber 0 21 20 Silica⁵ 112 74 0Silica⁶ 0 0 95 Carbon Black 0 5 5 Silane⁷ 11.2 0 0 Silane⁸ 0 5.92 6Silane⁹ 2 0 0 Traction resin¹⁰ 12 0 0 Traction resin¹¹ 0 4 0 Tractionresin¹² 0 0 7 Liquid Polymer¹³ 0 7 4 TDAE oil¹⁴ 38.784 10.125 10.139Sunflower oil 0 6.3 10 ¹High cis Neodymium BR, Tg = −106° C. obtainedfrom Goodyear Chemical Division as BUDENE1223 ²Solution polymerized SBRwith styrene content of 40% and 1,2-vinyl content of 14.4%, Tg = −34°C., extended with 37.5 phr TDAE oil, obtained from Trinseo as SESLR6430. ³Solution polymerized SBR with styrene content of 34% and1,2-vinyl content of 38%, Tg = −28° C. extended with 37.5 phr TDAE oil,obtained as Tufdene E680 from JSR. ⁴Solution polymerized SBR withstyrene content of 21% and 1,2-vinyl content of 50%, Tg = −23° C.obtained from Trinseo as SLR4602. ⁵Zeosil Premium 200MP from Solvay⁶HSCTS Agilon454G (surface area 200 m2/g CTAB) from PPG ⁷TESPD typesilane coupling agent, as Si266 from Evonik.⁸S-octanoylmercaptopropyltriethoxysilane, as NXT* from Momentive ⁹TESPDtype silane coupling agent, 50% on carbon black as X50S from Evonik.¹⁰Styrene and alpha-methylstyrene resin, Tg = 39° C., obtained asSylvares SA85 from Arizona Chemicals. ¹¹Polyterpene resin, Tg = 70° C.,obtained as Sylvares TRB 115 from Arizona Chemicals. ¹²Terpene phenolresin, Tg = 110° C., obtained as YS Polyster T160 from YasuharaChemical. ¹³Polybutadiene end functionalized with hydroxyl groups, Mw =2100, Tg = −35° C., as Krasol LBH-P 2000 from Cray Valley ¹⁴Includesextension oil and added oil

TABLE 2 Reference Example 1 Example 2 RR 100 120 113 Wet braking 100 9097 Dry braking 100 98 100

As can be seen in Table 2, the overall compromise of wet braking, drybraking and rolling resistance is improved with the compounds usinghydroxy-terminated polybutadiene, HSCTS and terpene phenol resin.

The invention claimed is:
 1. A pneumatic tire comprising a tread, thetread comprising a rubber composition comprising a diene elastomer, apre-hydrophobated precipitated silica, a blocked mercaptosilane, aresin, and a low molecular weight polybutadiene functionalized with ahydroxyl functional group, wherein the pre-hydrophobated precipitatedsilica is hydrophobated prior to its addition the rubber composition bytreatment with at least one silane selected from the group consisting ofalkylsilanes, alkoxysilanes, organoalkoxysilyl polysulfides andorganomercaptoalkoxysilanes and optionally at least one dispersing aidselected from the group consisting of fatty acids, diethylene glycols,polyethylene glycols, fatty acid esters of hydrogenated ornon-hydrogenated C₅ or C₆ sugars, polyoxyethylene derivatives of fattyacid esters of hydrogenated or non-hydrogenated C₅ or C₆ sugars.
 2. Thepneumatic tire of claim 1, wherein the rubber composition comprises asthe diene elastomer, from 70 to 90 phr of at least one styrene-butadienerubber, and from 10 to 30 phr of a natural rubber or syntheticpolyisoprene.
 3. The pneumatic tire of claim 2, wherein thestyrene-butadiene rubber comprises a first styrene-butadiene rubber anda second styrene-butadiene rubber.
 4. The pneumatic tire of claim 3,wherein at least one of the first and second styrene-butadiene rubber isfunctionalized with a alkoxysilane group and at least one group selectedfrom sulfur containing functional group and primary amino functionalgroups.
 5. The pneumatic tire of claim 1, wherein the rubber compositioncomprises as the diene elastomer A) from 40 to 60 phr of a firststyrene-butadiene rubber having a Tg ranging from −70° C. to −5° C. andfunctionalized with a alkoxysilane group and sulfur containingfunctional group, B) from 20 to 30 phr of a second styrene-butadienerubber containing from 25 to 45 percent by weight of styrene, a vinyl1,2 content of 20 to 60 percent by weight based on the rubber weight, aTg of from −30° C. to −5° C., and C) from 10 to 30 phr of a naturalrubber or synthetic polyisoprene.
 6. The pneumatic tire of claim 1,wherein the rubber composition comprises as the blocked mercaptosilanefrom 3 to 20 phr of S-octanoylmercaptopropyltriethoxysilane.
 7. Thepneumatic tire of claim 1, wherein the low molecular weightpolybutadiene functionalized with a hydroxyl functional group is presentin an amount ranging from 3 to 30 phr and has a molecular weight Mwranging from 1000 to 25000 g/gmol and a Tg ranging from −50° C. to −20°C.
 8. The pneumatic tire of claim 1, wherein the rubber compositionfurther comprises a processing oil, and the sum of the amounts of theresin, the low molecular weight polybutadiene, and the processing oilranges from 10 to 45 phr.
 9. The pneumatic tire of claim 1, wherein therubber composition comprises from 50 to 150 phr of pre-hydrophobatedsilica.
 10. The pneumatic tire of claim 1, wherein the first rubbercomposition comprises from 1 to 10 phr of carbon black.
 11. Thepneumatic tire of claim 1, wherein the rubber composition includes from3 to 25 phr of the resin.
 12. The pneumatic tire of claim 1, wherein theresin is selected from the group consisting of terpene-phenol resins andhydrocarbon resins.
 13. A pneumatic tire comprising a tread, the treadcomprising a rubber composition comprising 100 phr of elastomersconsisting of the following A, B, C: A) from 40 to 60 phr of a firststyrene-butadiene rubber having a Tg ranging from −40° C. to −10° C. andfunctionalized with a alkoxysilane group and sulfur containingfunctional group, B) from 20 to 30 phr of a second styrene-butadienerubber containing from 25 to 45 percent by weight of styrene, a vinyl1,2 content of 20 to 60 percent by weight based on the rubber weight, aTg of from −30° C. to −5° C., and C) from 10 to 30 phr of a naturalrubber or synthetic polyisoprene; 3 to 20 phr ofS-octanoylmercaptopropyltriethoxysilane; 3 to 25 phr of a resin; 3 to 30phr of a low molecular weight polybutadiene functionalized with ahydroxyl functional group and having a molecular weight Mw ranging from1500 to 2500 g/mol and a Tg ranging from −50 to −20° C.; From 10 to 25phr of a processing oil; 50 to 150 phr of pre-hydrophobated silica; and1 to 10 phr of carbon black; wherein the sum of the amounts of theterpene-phenol resin, the processing oil, and the low molecular weightpolybutadiene ranges from 10 to 45 phr.